The present invention is directed to the control of an internal combustion engine, and more particularly to a method of determining and preserving information of interest regarding the engine soak time.
Many engine control algorithms, including fuel control algorithms and diagnostic algorithms, require a reasonably accurate assessment of the engine soak time to achieve optimal and reliable operation. While it is possible to simply measure the time since the last electrical power-down or key-off event, such information is insufficient for many purposes, and may actually misrepresent the engine soak time if one or more short engine cycles occur. Accordingly, many algorithms infer soak time by measuring changes in the engine coolant or ambient temperatures, even though such information is only indirectly related to soak time. See for example, the U.S. Pat. No. 5,566,546 to Rumpsa et al., which defines soak time as the time between when the engine is turned off and then restarted. Instead of measuring the soak time with a timer, Rumpsa et al. infer the soak time based on measured engine and air temperatures at engine turn-off and at engine re-starting. And even these techniques become useless if there is a failure of the temperature sensor. Accordingly, what is needed is a simple and cost-effective method of reliably and accurately determining and preserving time intervals that are related to the engine soak time, referred to herein as soak-related time intervals.
The present invention is directed to an improved method of determining and preserving soak-related time intervals of an internal combustion engine by resetting a continuous timer at the end of each engine run period, and reading the value of the continuous timer at specified events during an ignition key cycle. According to the preferred embodiment, the method measures and preserves the engine soak time preceding the current and previous ignition key cycles, and determines an extended soak time that includes intervals of engine operation that are too short to significantly affect the engine thermal and fuel vapor purge conditions. The determined soak-related times are then utilized by various engine control and/or diagnostic algorithms, such as a hot re-start fuel control algorithm.